1. Field of the Invention
The invention relates to a resonator composed of a conducting metal ring surrounding a cylindrical dielectric core material which can be incorporated into multi-cavity filters for frequency separation.
2. Description of Related Art
Dielectric resonator filters are a class of stable microwave filters that are frequently used in radar and communications systems. Dielectric resonators are often utilized in filter circuits because of an intrinsically high Q value. These characteristics allow a filter employing a dielectric resonator to have excellent frequency stability with only a small amount of frequency drift over a wide range of temperatures and environmental conditions. The Q value of a dielectric resonator is defined as the ratio between the energy stored per cycle to the energy dissipated per cycle.
Dielectric resonators are typically made of a ceramic type material having a high dielectric constant (∈r=20 to 90) and a low dissipative loss. These characteristics allow the dielectric resonator to store energy with relatively little internal energy dissipation. This corresponds to a high Q value.
One significant limitation of the practical use of dielectric resonator filters is the cost of the dielectric itself. The cost of a typical prior art 6xe2x80x3 ceramic dielectric cylindrical resonator can cost three hundred dollars or more. In addition, the size of the resonator substantially increases the size of any multi-cavity filter in which it might be employed.
The following patents are generally representative of typical prior art dielectric resonators: U.S. Pat. Nos. 4,757,289; 5,140,285; and, 5,356,844.
Resonators are typically employed in filters for the wireless communication industry. Such filters typically include a plurality of resonators located in adjacent cavities and coupled to each other through a variety of different means. One coupling mechanism known in the prior art is the use of a tunable iris as described in U.S. Pat. No. 5,220,300 entitled xe2x80x9cRESONATOR FILTERS WITH WIDE STOPBANDSxe2x80x9d and issued on Jun. 15, 1993 and assigned by Richard V. Snyder to RS Microwave Company, Inc., the entire contents and substance of which is incorporated herein by reference. Other cutoff means are also known, but few are known that would be suitable for composite resonators such as described in this disclosure.
What is clearly missing in the prior art, therefore, is a relatively inexpensive resonator, of reasonably small size, that can be used in a multi-cavity filter structure without appreciable loss in performance.
Briefly described, the invention comprises a composite resonator preferably including a cylindrical ceramic core and an exterior metal layer that surrounds most of the exterior circumference of the core and wherein the resonator resonates in substantially bound modes. This composite configuration is used to provide resonant frequencies lower than can be obtained using the same volume of dielectric alone and with higher unloaded Q than can be obtained using the same volume of metal imbedded into a cavity and used as a resonator. An inexpensive metal, such as aluminum, can be substituted for more than half of the dielectric and still form a resonator with substantially equivalent resonance properties.
According to alternative embodiments of the invention, the resonators are incorporated into spectrum filters for separation of frequencies. As contrasted to .conventional prior art implementations, the new technique achieves similar, or better, electrical performance; similar, or reduced, size; and significantly reduced cost for applications in the frequency range below 2.5 Ghz, thus including PC, wireless, AMPS and GSM applications, as well as a myriad of other applications in this frequency range. With spectrum currently selling for up to $45.00 per Hz, filters are very valuable for providing users the opportunity to utilize all spectrum available. Yet, the cost of the filters must ultimately be borne by the users, so reductions in cost are important to commercial applications. The present invention in its various embodiments contributes to such a reduction in cost.
These and other features of the invention will be more fully understood by reference to the following drawings.